A thermoplastic resin is used as a material to form various kinds of molded products because of moldability thereof. However, the thermoplastic resin is softened at around the melting point thereof, and accordingly a molded product made of a thermoplastic resin having high moldability may be used only at a low temperature. Meanwhile, an engineering plastic, which has characteristically high strength, generally has a high melting point. However, the engineering plastic requires to be heated at a high temperature for molding, and thus special attentions have to be paid to its workability and safety on the molding.
Thus, if a thermoplastic resin can be molded at a relatively low temperature where processing of the resin is relatively easy and if the heat resistance and strength of the resin can be increased by a certain treatment after molding, the resin provides both excellent workability in molding and high material functions.
A general method to provide a thermoplastic resin having both excellent workability in molding and high material functions is crosslinking among polymer molecules. Examples of the crosslinking include electron-beam crosslinking and silane crosslinking (see Patent Literatures 1-4).
In the electron-beam crosslinking, as shown in Patent Literature 1, a resin such as a polyethylene is molded into a desired shape, and then the molded product is irradiated with an electron beam, whereby polymer chains are crosslinked radically. By the crosslinking, heat resistance and strength are imparted to the resin. However, this method requires a large-scale equipment including an electron beam source and an adequate electron beam shield.
In the silane crosslinking, as shown in Patent Literatures 2-4, a resin composition prepared by melt-kneading of ingredients such a polyolefin-based polymer and a silanol catalyst is kneaded and extrusion-molded with a silane-crosslinkable polyolefin, to provide a molded product. Then, the molded product is brought into contact with moisture and thus is crosslinked. However, including the contact of the molded product with moisture, this method can not be applied to a system containing a material that may be deteriorated by moisture. Further, curing of the composition proceeds so slowly in the method that the method is not suitably used for a short-time treatment.
Ultraviolet-irradiation crosslinking is a method that completes crosslinking in a short time like the electron-beam crosslinking but does not require a large-scale equipment. In this method, a compound that is radically polymerized by ultraviolet irradiation (i.e., an ultraviolet crosslinking agent) is dispersed in a composition containing a polymer, and radicals generated by ultraviolet irradiation crosslinks polymer molecules adjacent to each other directly or crosslinks polymer molecules via a radical-polymerized product of the ultraviolet crosslinking agent.
Examples of a thermoplastic resin composition to be molded through the ultraviolet-irradiation crosslinking include a composition containing a thermoplastic resin, a (meth)acrylate derivative, and an ultraviolet crosslinking agent. If the composition can be hot-molded at a molding temperature of the thermoplastic resin to provide a molded product, and if the heat resistance and strength of the molded product can be increased by ultraviolet crosslinking through irradiation with ultraviolet, the composition provides both excellent workability in molding and high material properties.